Azo yellow pigments are a class of colorants that are tinctorially strong and relatively inexpensive. Monoazo yellow pigments show good lightfastness and are particularly useful as coloring agents for paints and some printing inks. In plastics, however, monoazo yellow pigments tend to undesirably bloom and discolor at high temperatures, so their use is disfavored. Disazo (diarylide) yellow pigments are much stronger tinctorially and show a much lower tendency to bloom and discolor at high temperatures. However, R. Az et al reports in Dyes and Pigments, 15, 1 (1991), that diarylide yellows are degraded to potentially carcinogenic by-products (e.g., 3,3'-dichlorobenzidine) in plastics processed above 200.degree. C., a temperature lower than that used in processing most plastics (many plastics are processed at temperatures from 250.degree. C. to 330.degree. C.). In attempts to overcome these disadvantages, azo pigment manufacturers have actively pursued development of new metallized azo yellows, analogous to the metallized azo red pigments that show good high temperature stability in plastics.
Two recent patents involve coupling diazotized 2-amino-5-chloro-4-methyl (or ethyl) benzene-1 -sulfonic acid into 1-substituted phenyl-3-methyl-5-pyrazolones and metallizing with calcium or strontium. Specifically, U.S. Pat. No. 5,669,967 to Hays involves coupling into 1-(2-chlorophenyl)-3-methyl-5-pyrazolone, while U.S. Pat. No. 5,746,821 also to Hays involves coupling into 1-(4-methylphenyl)-3-methyl-5-pyrazolone.
In the past two decades, there has been notable development activity in metallized azo yellow pigments involving coupling various diazotized 2-aminobenzene-1 -sulfonic acid derivatives into coupler 1-(3'-sulfophenyl)-3-methyl-5-pyrazolone (hereinafter 3-sulfo-PMP) and metallizing. For instance, German Offenlegungsschrift specification No. 2616981 (1977, BASF) relates to coupling the calcium salt of diazotized 2-amino-4,5-dichlorobenzene-1-sulfonic acid into 3-sulfo-PMP (Colour Index (C. I.) Pigment Yellow 183). U.S. Pat. No. 4,980,458 to Hari et al discloses the mixed sodium, ammonium or half-calcium salts of diazotized 2-amino-4,5-dichlorobenzene-1-sulfonic acid coupled into 3-sulfo-PMP. U.S. Pat. No. 4,594,411 to Henning discloses the calcium salts of diazotized 2-amino-5-acetylaminobenzene-1-sulfonic acid, 2-amino-5-methoxybenzene-1-sulfonic acid, 2-amino-5-methylbenzene- 1 -sulfonic acid or 2-amino-5-chlorobenzene-1-sulfonic acid coupled into 3-sulfo-PMP. U.S. Pat. No. 5,047,517 to Deucker discloses the calcium salt of diazotized 2-amino-4-chloro-5-methylbenzene-1-sulfonic acid coupled into 3-sulfo-PMP (C. I. Pigment Yellow 191). German Offenlegungsschrift 4111348 (1992, Sandoz) discloses the strontium salt of diazotized 2-amino-4-chloro-5-methylbenzene-1-sulfonic acid coupled into 3-sulfo-PMP. European Patent 0841 367A2 (1998, Clariant Finance Ltd.) relates to the use of this pigment in plastic packaging material.
U.S. Pat. No. 5,733,365 to Halko et al discloses treatment of titanium dioxide with di- C.sub.2-20 alkyl sulfo-succinates, but describes Examples with only the well-known dioctyl sulfosuccinate.
Although a few of the metallized azo yellows made from 3-sulfo-PMP (e.g., C. I. Pigment Yellow 191) have been commercialized and show good fastness to heat and light in plastics, they suffer from low tinctorial strength (W. Herbst and K. Hunger, Industrial Organic Pigments, VCH, New York, 1993, pp. 236-237). For example, the pigment concentration required for coloring high density polyethylene containing 1% titanium dioxide to 1/3 Standard Depth of Shade is 0.32% for P.Y. 191 vs. only 0.08% for diarylide yellow P.Y. 83 (ibid, p. 255). Plastics processors using diarylide yellows are particularly conscious of the increased costs of using the higher concentrations of metallized azo yellows required for coloring plastics to the required shades. Thus, there is a need for metallized azo yellow pigments that show higher strength, as well as good heat stability and good lightfastness.